But the companies said there were no plans for layoffs affecting Midwest Generation's almost 1,000 employees in Illinois.

In bankruptcy petitions filed in U.S. Bankruptcy Court Chicago, Edison Mission Energy listed $5.13 billion in assets and $5.1 billion in debt. The Santa Ana, Calif.-based company blamed its financial troubles on low power prices, high fuel costs and the need to retrofit coal-fired power plants to meet environmental regulations.

Edison Mission Energy is owned by the California utility Edison International. Edison Mission Energy President Pedro Pizarro said the hope is to emerge from bankruptcy as a separate company independent of Edison International.

"This is an important first step in the process to reduce our debt, enhance our liquidity profile and position EME for continued operation and future success while preserving our ability to generate power safely and reliably at our electric facilities across the country," he said in a news release. "Throughout this process, business operations will continue in the normal course, and we will continue to support our customers, suppliers and employees."

Edison Mission Energy said the bankruptcy filing shouldn't have any immediate effect on the operations of either company, which include Midwest Generation plants in Joliet, Pekin, Waukegan and Romeoville.

The business information website Hoover's says Midwest has about 960 employees. According to its website, Edison Mission Energy through its subsidiaries owns or leased more than 40 power plants in California, Pennsylvania, Iowa, Illinois and other states capable of producing more than 10,000 megawatts of power. The company also has a plant in Turkey.

In the news release, the companies said they plan to separate from Edison International by the end of 2014.